Real-time discovery of AT2020xnd: a fast, luminous ultraviolet transient with minimal radioactive ejecta

Abstract

The many unusual properties of the enigmatic AT2018cow suggested that at least some subset of the empirical class of fast blue optical transients (FBOTs) represents a genuinely new astrophysical phenomenon. Unfortunately, the intrinsic rarity and fleeting nature of these events have made it difficult to identify additional examples early enough to acquire the observations necessary to constrain theoretical models. We present here the Zwicky Transient Facility discovery of AT2020xnd (ZTF20acigmel, the “Camel”) at z = 0.243, the first unambiguous AT2018cow analog to be found and confirmed in real time. AT2018cow and AT2020xnd share all key observational properties: a fast optical rise, sustained high photospheric temperature, absence of a second peak attributable to ejection of a radioactively-heated stellar envelope, extremely luminous radio, millimetre, and X-ray emission, and a dwarf-galaxy host. This supports the argument that AT2018cow-like events represent a distinct phenomenon from slower-evolving radio-quiet supernovae, likely requiring a different progenitor or a different central engine. The sample properties of the four known members of this class to date disfavour tidal disruption models but are consistent with the alternative model of an accretion powered jet following the direct collapse of a massive star to a black hole. Contextual filtering of alert streams combined with rapid photometric verification using multi-band imaging provides an efficient way to identify future members of this class, even at high redshift.

AT 2018lqh and the Nature of the Emerging Population of Day-scale Duration Optical Transients

“…However, in such cases, we expect to detect higher-ionization species (e.g., Chevalier & Fransson 1994;Gal-Yam et al 2014;Yaron et al 2017). We note that this scenario can explain, as discussed in the literature, objects like AT 2018cow (Perley et al 2019) and AT 2020xnd (Perley et al 2021). In these cases, intermediatewidth emission lines are indeed observed.…”

“…Drout et al (2014), Arcavi et al (2016), and, more recently, Pursiainen et al (2018) and Ho et al (2021) reported several examples of transients with a duration above a half-maximum light of ∼3 to 12 days. Additional examples include, among others, PTF 09uj (Ofek et al 2010), AT 2017gfo (Abbott et al 2017b), AT 2018cow (Perley et al 2019), and AT 2020xnd (Perley et al 2021). These transients span a wide range of peak absolute magnitudes, from −16 to −21.…”

A common envelope jets supernova (CEJSN) impostor scenario for fast blue optical transients

“…AT2018cow-like FBOTs tend to occur in star-forming galaxies, namely, come from massive stars (e.g., Prentice et al 2018;Perley et al 2019;Lyman et al 2020;Perley et al 2021). Soker, Grichener, & Gilkis (2019) noted that from their population synthesis study Mapelli et al (2018) find that in the local Universe NS-NS mergers tend to occur shortly after star formation.…”

Cosmological Fast Optical Transients with the Zwicky Transient Facility: A Search for Dirty Fireballs

“…Every night, the ZTF alert stream is filtered by several independent pipelines to identify young or fast-evolving transients, such as ZTFReST (Andreoni et al 2020(Andreoni et al , 2021. In this paper we focus on events discovered via the approach described in Ho et al (2020b) and Perley et al (2021c). In short, basic cuts are applied to remove artifacts, asteroids, and stellar flares.…”

“…Measurements of superluminous supernovae (SLSNe), Type Ia supernovae (SN Ia), and most core-collapse supernovae (SNe) are from the ZTF Bright Transient Survey(Fremling et al 2020;. Measurements of short-duration core-collapse SNe and AT 2018cow-like transients are from dedicated searches for fast-evolving transients(Prentice et al 2018;Perley et al 2019bPerley et al , 2021cHo et al 2020cHo et al , 2021cYao et al 2021e). For reference we also show the timescale and luminosity of the optical emission from GW170817/AT 2017gfo(Coulter et al 2017;Cowperthwaite et al 2017;Kasliwal et al 2017;Drout et al 2017;Villar et al 2017b) and of iPTF14yb (the only optically discovered afterglow prior to ZTF with a redshift measurement;Cenko et al 2015).…”